1. Field of the Invention
This invention relates to a process for the inspection of monocrystalline material for the presence of precipitations of impurities, whereby discrete crystal regions are irradiated with wave or particle rays, the intensity distribution of rays diffracted under Bragg conditions is recorded, and a determination of the static Debye-Waller factor is made by fitting the recorded intensity values to a specified theoretical function.
2. Background of the Invention
Since the control and optimizing of the manufacture and processing of single crystals play an important role in many industrial applications, great significance is attached to processes which make it possible for the manufactured crystals to be inspected and the deviations from the ideal crystal structure detected. A typical use of such processes is the inspection of Silicon (Si) wafers which are to be reprocessed for use in the semiconductor and microelectronics industries.
Si wafers are generally in the form of monocrystalline discs with typical dimensions of approximately 0.4 mm thick and 200 mm in diameter. The wafers obtained, after the crystal drawing, contain an oxygen component on the order of magnitude of 10 ppm, with the exception of the active surface layer, approximately 20 mm thick, which is practically oxygen-free. During reprocessing the wafers are heated during tempering, whereby at approximately 700.degree. C., the previously atomic oxygen tends to form SiO.sub.2 molecules. The SiO.sub.2 molecules can substitute for Si atoms in the crystal lattice, which causes local distortions of the lattice structures.
It has been shown that the distortion of the lattice has an attractive effect on additional SiO.sub.2 molecules, which accumulate in the plane structure. In this manner, thin platelets of SiO.sub.2 precipitations form, which causes stresses in the crystal. These precipitations also have the characteristic of gettering sinks. Gettering is the process by which existing defects are annihilated and impurities are removed. More specifically, intrinsic gettering occurs in Si crystals when the precipitations attract foreign atoms internally; in particular metal atoms such as Ag, Cu, etc., which originate from impurities in chemicals required during the manufacturing process for semiconductor elements, e.g. etching fluids. As a result of the intrinsic gettering, the yield during the manufacture of micro-electronic semiconductor components is significantly increased. A prerequisite of intrinsic gettering is that the intrinsic gettering layer is underneath the subsequently electrically active surface layer of the wafer. The gettering layer helps purify the electrically active surface layer.
The stresses caused by precipitations in the crystal lattice are reduced at the elevated temperatures of more than 1000.degree. C., which elevated temperatures occur during the subsequent process stages, and result in stacking errors. The stacking errors cause extensive distortion fields, which extend into the oxygen-free surface layer. It is desirable, to "suck out" impurities from the surface layer and to fix the stacking errors in the lower crystal layers. Again, the intent is to purify the electrically active surface layer. It is therefore desirable to design the treatment processes for the wafer so that there is a uniform distribution and homogenization of the SiO.sub.2 precipitations, to guarantee the most uniformly possible intrinsic gettering and thus to prevent disruptions of the electrically active surface layer.
Accordingly, there has long been a demand for a process by way of which the impurity precipitations can be made observable and measurable, in terms of density, size, and distribution, in the wafer. This is true in particular for a process which can be performed repeatedly in individual process steps or even in situ, i.e. during tempering of the wafer in a furnace.
The Japanese publication "Japanese Journal of Applied Physics, Volume 27, No. 8, June 1988, Pages 1081 to 1087 (S. Iida et al: Measurement and Analysis of the Static Debye-Waller Factor of Cz-Silicon with Small Oxygen Precipitates) discloses a method of the type described above, which is suitable for laboratory-scale tests on Si specimens for SiO.sub.2 precipitations both of large extent and low density, and also for such micro-precipitations whose dimensions are small in relation to the resolution of the X-ray diffraction intensity distribution recorded. The intensity distribution is thereby recorded photographically, and the blackening of the negative can be evaluated, for example, with a microdensitometer. For this purpose, specimens and sharply collimated beams must be oriented in a fixed manner, and the specimens and the negative to be exposed must be at the smallest possible distance from one another.